cgrp->root->release_agent_path is protected by both cgroup_mutex andrelease_agent_path_lock and readers can hold either one. Thedual-locking scheme was introduced while breaking a locking dependencyissue around cgroup_mutex but doesn't make sense anymore given thatthe only remaining reader which uses cgroup_mutex iscgroup1_releaes_agent().

This patch updates cgroup1_release_agent() to userelease_agent_path_lock so that release_agent_path is always protectedonly by release_agent_path_lock.

While at it, convert strlen() based empty string checks to directtests on the first character as suggested by Linus.